Hello,
It's not
AdwCleaner is a portable scanner/remediation tool which doesn't need to be installed on the system. It's one self-contained and low on resources executable (whereas MBAM requires to be installed and is a more heavy).
It's also more focused on PUP/Adware/Toolbars and related infections, thus it's able to scan and remove them faster than MBAM which is able to catch more threats and not only toolbars/adware/... .
So it really depends on your needs here, but both are complementary and a lot of people are doing that.
Regards,

Hello,
It's actually a feature
If you launch AdwCleaner, click on Tools > Options you'll see a list of additional actions you can apply during the cleaning process. Hence even if the scan doesn't list anything malicious you may still want to apply those additional actions.
Regards,